Rod-like liquid crystal materials having positive birefringence (Δn>0, Δn=ne−no) are used in TFT liquid crystal display at present. Anisotropy of the rod-like liquid crystal materials will cause light leak of the display and result in a narrower viewing angle. In order to improve the viewing angle of the display, it is an effective method to compensate the optical path difference of line polarized light after passing through a liquid crystal cell by using an optical compensation film having negative birefringence (Δn<0). The optical compensation film materials reported currently comprise polymer films uniaxially or biaxially stretched, main chain liquid crystal macromolecules, macromolecules having rigid chain unit, photopolymerisable rod-like liquid crystal macromolecules and discotic liquid crystal compounds.
The birefringence of the triphenylene discotic liquid crystal having optically uniaxial nematic phase is negative, and its absolute value is close to that of the rod-like liquid crystal usually used in liquid crystal display. Therefore, the triphenylene discotic liquid crystal having optically uniaxial nematic phase can be used as an optical compensation film (cell) material to compensate the optical path difference of line polarized light after passing through a liquid crystal cell.
The use of hexaarylate of triphenylene compound having a nematic phase as optical compensation film material was reported in U.S. Pat. No. 7,495,730, U.S. Pat. No. 7,443,474 and U.S. Pat. No. 5,518,783. Such type of materials already reported mostly have a higher melting point (of above 100) and a clearing point of about 200. When used for preparing an optical compensation film, the material has to be heated to a temperature above the clearing point thereof, and then cooled slowly to control the orientation of the discotic molecules. Therefore, while ensuring that the liquid crystal phase temperature is in a suitable range, reducing the clearing point of the material can save energy and simplify preparation process. The melting point of a material must be lower than room temperature when used as an optical compensation cell material.
Therefore, there is a need for providing a liquid crystal material having a suitable temperature range of liquid crystal phase and a lower clearing point.